This invention relates generally to electrophotographic printing, and more particularly concerns an illumination system employed therein.
In the process of electrophotographic printing, a photoconductive member is uniformly charged. The charged photoconductive member is irradiated with a light image of an original document. The light image is projected onto the charged photoconductive surface to selectively discharge the charge recording an electrostatic latent image thereon. During development, toner particles are electrostatically attracted to the latent image rendering it visible. Subsequently, the toner powder image is transferred from the photoconductive member to a sheet of support material. The powder image is permanently affixed to the sheet of support material producing a copy of the original document thereon. This process is described in greater detail in U.S. Pat. No. 2,287,691 issued to Carlson in 1942.
In the process of color electrophotographic printing, the optical system forms successive filtered light images of the original document. The filtered or single color light image exposes the charged photoconductive member recording a single color electrostatic latent image thereon. The single color electrostatic latent image is developed with toner particles complementary in color thereto. These toner powder images are transferred from the electrostatic latent image to a sheet of support material. This process is repeated a plurality of cycles with differently colored light images and their respective complementary colored toner particles. Each single color toner powder image is transferred to the sheet of support material in superimposed registration with the prior toner powder image. In this manner, a multi-layered toner powder image is produced on the sheet of support material. This multi-layered toner powder image is then permanently affixed thereto forming a color copy.
In all of the foregoing processes, the function of the exposure or optical system is to illuminate the original document forming a light image thereof which irradiates the charged portion of the photoconductive member. Various techniques have been developed for producing a light image. In one technique, the entire original document is exposed substantially simultaneously. This technique is known as a full frame exposure system. An alternate approach requires the movement of the light source across the original document in synchronism with the motion of the photoconductive member. Thus, a flowing light image is formed which is projected onto the moving photoconductive member. However, in all instances, the light source is cycled on and off at the line frequency. This produces a strobing effect on the resultant copy. Strobing on the copy is a significant problem and manifests itself as a series of developed lines having a spatial frequency equal to the operating frequency of the light source. Generally, strobing will occur with conventional fluorescent lamps. However, this problem is usually overcome by the retention of the phosphor and the required decay time as well as optical de-focusing. In systems using Xenon or gas discharge sources there is frequently little or no retention time. Thus, the light sources cycle on or off at the operating frequency producing copies having a strobing effect.
Accordingly, it is the primary object of the present invention to improve the illumination system of an electrophotographic printing machine by minimizing the effects of strobing on the resultant copy.